Optical communications systems at very high bit rates (typically from 10 to 100 Gbit/s and more) are currently being studied. In these systems, a plurality of channels are transmitted, and each of them conveys information represented by a succession of 0 and 1 pulses. The pulses of a channel are transmitted at a relatively low bit rate (up to 10 Gbit/s) and between two successive pulses of a channel there are inserted, in a pre-determined sequence, the pulses relating to the other channels, transmitted at the same rate. This multiplexing method, well known from electronic signal technique, is named in the case at hand "Optical Time Division Multiplexing", commonly known with the acronym OTDM.
It is evident that, to fully exploit the capacity of the transmission medium with the method described, it is desirable that the pulses be as narrow as possible in order to avoid interference between the channels and to allow the correct demultiplexing at the receiving side. Actually, at those rates, demultiplexing must be performed completely optically. Some of the techniques proposed for this purpose, which exploit the so-called Four Wave Mixing (FWM) or the Kerr effect in optical Fibers (Nonlinear Optical Loop Mirror, NOLM) and are based on the overlapping between the pulse of the channel to be extracted and a pump pulse within that Fiber (which acts as a nonlinear medium), require that the two pulses remain overlapping as long as possible during the travel along the Fiber. Furthermore the pulses must have such shape and band characteristics that the pulses themselves propagate with as little distortion as possible. This requirement is generally expressed by saying that the pulses must be "transform limited", this expression meaning that the product between the duration or full width at half maximum (FWHM) .DELTA.t and the bandwidth .DELTA..nu. must have a certain value, corresponding to the theoretical minimum, which depends on the pulse shape. In particular, since the pulses that are most commonly used and that have yielded the best results in transmission are Gaussian and hyperbolic secant pulses, the term "transform limited" is used to indicate pulses where the product .DELTA.t.multidot..DELTA..nu. takes a value that corresponds or is close to that of the Gaussian pulse or the hyperbolic secant pulse (0.441 and respectively 0.314).
To generate pulses with these characteristics, it has been proposed to utilize the direct modulation of a semiconductor laser by means of pulses of such duration as to excite only the first peak of the laser relaxation oscillations (gain switching technique). In that condition the pulses emitted by the laser exhibit, because of the modulation, a high chirp and therefore, before being utilized, they are made to propagate in an optical Fiber with such dispersion characteristics as to compensate the phase distortion produced by said chirp. This technique of generating ultrashort, transform-limited optical pulses is described for example by H. F. Liu et al. in "Generation of an extremely short singlemode pulse (2 ps) by Fiber compression of a gain-switched pulse from a 1.3 .mu.m distributed feedback laser diode", Applied Physics Letters 59 (11), 9 Sep. 1991, by K. A. Ahmed et al. in "Nearly transform-limited (3-6 ps) generation from gain-switched 1.55 .mu.m distributed feedback laser by using Fiber compression technique", Electronics Letters, vol. 29, no. 1, 7 January 1993, or yet by J. T. Ong et al. in "Subpicosecond Soliton Compression of Gain-Switched Diode Laser Pulses Using an Erbium-Doped Fiber Amplifier", IEEE Journal of Quantum Electronics, vol. 29, no. 6, Jun. 1993.
The system described in the first of the above articles generates a pulse that yields a product .DELTA.t.multidot..DELTA..nu. within the desired range, but utilizes a source whose wavelength (1.3 .mu.m) does not coincide with the null-dispersion wavelength (.about.1.55 .mu.m) of the Fibers utilized in the optical demultiplexing systems proposed until now; moreover, the pulse is strongly affected by noise and it has a shape (Lorentzian pulse) that is not normally exploited for information transmission. The methods described in the second and third articles utilize sources at a wavelength of 1.55 .mu.m, as required for the subsequent demultiplexing, but originate pulses whose product .DELTA.t.multidot..DELTA..nu. is very far from the desired interval.